December 14, 2012

Rainforest Study Reveals Insects Outnumber Mammals 300 To 1

Until now, scientists have had a problem estimating the exact numbers of arthropods — a group that includes insects, arachnids, and crustaceans - on Earth, even though we know they comprise a majority of the multicellular species on the planet. Because of their small size, frequent movements, and inaccessible habitats, quantifying their abundance has been difficult despite the fact that they are the most numerous phylum on the planet.

A new study, led by Yves Basset from the Smithsonian Tropical Research Institute, provides an unprecedented level of detail regarding the diversity and distribution of arthropod species from the soil to the forest canopy in the rainforests of Panama. The findings of this study, recently published in the journal Science, show the arthropods outnumbering mammals — including humans — at a ratio of 312 to 1. They outnumber plants at a more modest scale of 17 to 1.

Basset says there are simple explanations for the sheer number of arthropods.

"They are small and can make a living out of nearly everything, including other arthropods, decomposing matter, plant tissues, etc." He also points out that bug larvae rarely compete with adults of the same species since they feed on different food sources.

The international team of scientists working on Project IBISCA-Panama sorted, sampled, catalogued and finally estimated that a 6,000-hectare (23.1 square miles) forest — the San Lorenzo forest reserve - hosts a total of approximately 25,000 arthropod species. This number vastly outnumbers that of better-studied organisms.

"Arthropods are important in all the functions of the forest: pollination, early decomposition, [consumption] of leaves in the forest, [providing] nutrients in soil, and regeneration of [the] forest," says id Basset, scientific coordinator of the Institute's CFTS Arthropod Initiative.

Though arthropods tend to thrive in tropical settings, the team believes this forest is typical of forest arthropod population. Basset told Discovery News that the ratios might even skew farther in favor of the arthropods in an urban setting.

"The mammal fauna in cities is rather depleted, but not necessarily that of arthropods," he explained. "For example, a small urban park may not host many mammal species, because it may be a too small area to sustain species requirements, such as food and living space. However, let's say you have 10 species of trees in this park, then they may well support as many as 200 arthropod species, according to our data."

Humans tend to think of arthropods as detrimental as they can sometimes spread disease and destroy crops.

"But we forget that these represent only a few species in comparison to the whole of arthropod biodiversity," Basset said. "The majority of insects live in forests and are responsible for the maintenance of these forests via the different services of pollination, decomposition and herbivory. In addition, many arthropods are efficient predators or parasites that suppress the levels of herbivores."

Many arthropod species face extinction, Basset warned. Having an accurate count of existing species and understanding which are functionally redundant will allow conservation efforts to be focused in ways that will truly help.

"If we have a thousand species breaking up wood in the forest, we want to know if we could maybe simplify this with maybe 500 species ... Can we afford to lose them? Is the basic function of the forest affected?" Basset told National Geographic.

This study, which involved 102 scientists from 21 countries, collected and identified arthropods from all parts of the Panamanian rainforest — spending nearly 70 person, or trap, years of effort between 2003 and 2004.

"You need to be an expert in ants to identify them, and you need to be a butterfly expert to do the same. The diversity of the experts working on the project is just a reflection of the diversity of the arthropods themselves," Basset told National Geographic.

The team sampled the rainforest canopy from a construction crane, inflatable platforms, balloons, and climbing ropes through forest layers. They also crawled along the forest floor to sift soils, trap and bait arthropods. From 2004 to 2012, the team sorted and identified 130,000 arthropods into over 6,000 distinct species.

The team found that the areas of the forest with the most arthropod diversity also had the most plant diversity.

"It means if we want to save areas of high species richness, we may [want to] focus on sites with plant species [richness]," Tomas Roslin, professor at the University of Helsinki, Finland told National Geographic.

The team calculated that the rainforest reserve harbors in excess of 25,000 arthropod species by scaling up the diversity values gathered from twelve exhaustively sampled sites.

"This is a high number as it implies that for every species of vascular plant, bird or mammal in this forest, you will find 20, 83 and 312 species of arthropods, respectively," explains Basset. "If we are interested in conserving the diversity of life on Earth, we should start thinking about how best to conserve arthropods," added Roslin in a press statement.

"What surprised us the most was that more than half of all species could be found in a single hectare of the forest", said Basset.

"This is good news, as it means that to determine the species diversity of a tropical rainforest, we need not sample gigantic areas: a total of one hectare may suffice to get an idea of regional arthropod richness — provided that this total includes widely spaced plots representative of variation within the forest," said Roslin.

Not everyone agrees that this extrapolation is the right method to use, however. Terry Erwin, an entomologist at the Smithsonian Institution's National Museum of Natural History, warns against putting too much weight on the estimated number of species, adding that further surveys across the San Lorenzo forest could help to make the estimates of arthropod species diversity more accurate.

Erwin himself made predictions of 30 million insect species in 1982, using a very limited scope of study. He fogged one species of tree in a Panamanian tropical forest with insecticide, identifying the beetle species that dropped to the forest floor. Erwin estimated the number of tree beetles to ground beetles, and the number of beetles to insects to arrive at his final number of 30 million. Subsequent research by Andrew Hamilton of the University of Melbourne, using similar methods on several tree species in New Guinea, reduced this estimate to the current 6 million.

The different between these older studies and the new research is that the previous work used a subgroup of insects to predict overall numbers. The new work is all-encompassing, counting all types of arthropods in large sections of forest.

“What´s phenomenal about this new study is that they cut through all of the assumptions we used,” Hamilton told Nature News. “Rather than assuming that one taxon represents another, they looked at the whole community.” It is important to note, however, that the new study did not attempt to describe and name new species. They grouped the unknown species by basic characteristics for the sake of the survey.

"This study is exciting because they've taken a large team of people and used every technique available," he told Science Now. "But to take a little sample from one place and scale up, it's been critiqued and critiqued and it just doesn't work."

According to Basset, "Another exciting finding was that the diversity of both herbivorous and non-herbivorous arthropods could be accurately predicted from the diversity of plants."

"By focusing conservation efforts on floristically diverse sites, we may save a large fraction of arthropods under the same umbrella. Further, this strengthens past ideas that we should really be basing estimates of global species richness on the number of plant species," stresses Roslin.

The team found that test sites with more tree species contained more arthropod species and they built a model to predict arthropod diversity on the fact that for every species of tree or other vascular plant, there were approximately 20 species of arthropods. Since plants are much easier to survey than insects, Basset says this model will simplify the classification of arthropod diversity.

Again, Erwin disagrees. He currently collects in Ecuador, where insect diversity not only increases with the number of trees, but with the number of microhabitats formed by varying compositions of tree species.

Despite his past predictions, Erwin now refuses to put a number on global diversity.

“We all make that mistake,” he told Nature News. “The problem is that the Panamanian tropics look nothing like the forests in Ecuador, and they look nothing like the forests of Borneo or the Congo.”

Basset, Erwin and Hamilton do agree that the pursuit of pinning down diversity is worthwhile research, however.

"While we have assigned immense resources to mapping our genes, resolving sub-atomic structures and searching for extra-terrestrial life, we have invested much less in exploring with whom we share the Earth. Why such research should be run on a shoe string budget just escapes me," reflected Basset in the press statement.

Basset claims that the sheer scale of this project is a first in the field of tropical entomology.

"In the past we've had quite a few [similar projects] but only targeting a single group, like ants or butterflies. This is the first project where we're trying to sample representative of each group of arthropods," he told National Geographic.

He warns that conservation efforts are necessary, even with the richness of arthropod diversity that was discovered.

"I would be quick to point out that if we have so many species in half a hectare, that doesn't mean that they are able to subsist in only half a hectare. We can collect them because they pass through, but that doesn't mean that you could cut the forest to just one or two hectares and preserve the biodiversity [of the whole forest]," he says.

Basset gives compelling reasons for the conservation efforts, stating the arthropods represent a formidable, but untapped, reserve of DNA, genes and molecules -- again about 20 times more species-rich than plants from which we nevertheless get most of our medications. Who knows what may be concealed in these arthropod molecules and how we could use them? "We also need to discover most of these species/molecules before they disappear from Earth."

Out of the hypothesized 6 million species of arthropods, we have classified only 1 million.

"In this context, I have difficulties understanding the enthusiasm of the public for the search for extra-terrestrial life," Basset told Discovery News. "Are we not wasting dollars on a doomed quest, whereas with a fraction of these funds, we could easily, as our study indicates, unveil a substantial amount of the Earth´s biodiversity before it is too late?"